1. Field of the Invention
The present invention relates to a method for bonding a heatsink and to a semiconductor device with a heatsink. More specifically, the invention relates to a method for bonding a heatsink to a semiconductor device by using an adhesive, and to a semiconductor device having a heatsink bonded by the above method for bonding.
2. Description of the Related Art
In recent years, radiating the heat generated in a semiconductor device is becoming a problem accompanying the integration of semiconductor devices and an increase in the operation frequency. To radiate the heat of the semiconductor device, it is becoming a general practice to adhere a heat-radiating heatsink to the semiconductor device. To bond the heatsink to the semiconductor device, there is used an adhesive having a low heat resistance and a coefficient of thermal expansion comparable to those of the heatsink and the semiconductor device. However, a period of several hours up to about one day is required for the adhesive to exhibit a sufficiently large fixing strength for bonding the semiconductor device to the heatsink. So far, therefore, a double-sided adhesive tape is used to temporarily fix the heatsink to the semiconductor device until the adhesive exhibits a sufficiently large fixing strength.
Usually, however, the heat resistance of the double-sided adhesive tape is much larger than the heat resistance of the adhesive. Therefore, the heat-radiating characteristics decrease with an increase in the area of the double-sided adhesive tape that occupies the adhesion surface between the heatsink and the semiconductor device. For example, the double-sided adhesive tape used for temporarily fixing the heatsink has a heat resistance of about 0.3° C./W. On the other hand, a silicon adhesive used for bonding the heatsink to the semiconductor device has a heat resistance of about 0.01° C./W. It is therefore desired that the above double-sided adhesive tape has an area which is as small as possible.
Further, when there is used a highly viscous adhesive, it may happen that the fixing strength at the time of temporary fixing decreases as the adhesive applied onto the adhesion surface of the heatsink or of the semiconductor device spreads and flows in between the double-sided adhesive tape and the semiconductor device at the time when the heatsink is pushed onto the semiconductor device.
Further, in order to prevent the adhesive from flowing in between the double-sided adhesive tape and the semiconductor device, it can be contrived to apply the adhesive in a predetermined shape by taking into consideration how the adhesive spreads. In this case, however, an increased number of operation steps is required for applying the adhesive and, as a result, an increased cost is required for bonding the heatsink to the semiconductor device.
Therefore, it has been desired to develop a method for bonding the heatsink to the semiconductor device for improving the heat-radiating efficiency.